Trailing suction hopper dredgers (“TSHD”) are vessels which can be used to dredge at sea or in open water. TSHD's typically use a suction tube, one end of which can be lowered to the seabed and used to suck up solids such as sand, sludge, silt or sediment, mixed with water. The lower end of this suction tube can be provided with a suction head or a drag head. The solid material mixed with water is pumped through the suction tube into a hopper of the dredging vessel. The mixture is reduced in speed when in the dredger hopper, and this speed reduction allows for the settling of components suspended in the mixture. Excess water is then overflowed out of the hopper through an overflow to allow for more load capacity within the TSHD.
Water from the hopper flows into the overflow through an entry disk or directly into the overflow from the top. This overflow volume is then released via the bottom of the dredging vessel. Due to the generally vertical flow orientation of the overflow, the cross-sectional area of the overflow and the velocity, the overflow volume typically mixes with air. This mixture can reduce the specific weight of the mixture, causing the overflow mixture to stick to the ship's hull and the boundary flow of the underwater vessel. This mixture, which may also contain lighter particles which have not settled, and the interactions between the overflow volume released, the hull, propellers, speed of the vessel and currents; can form a plume in the wake of the dredging process. The settling of this mixture can then have an adverse effect on the local environment. The air in the flow also causes a resistance in the overflow, reducing the effective transport capacity of the overflow.
One method to combat this plume caused by suspended particles is disclosed in WO 2013/119107. A passive overflow device is used to drain away head water and flow it through a conduit to an outlet abutting the sea bottom to deliver the head water close to the sea bottom, thereby minimizing the influence on sea life. U.S. Pat. No. 3,975,842 discloses a system which also attempts to minimize the environmental effects by directing the overflow to the suction head to be used as the liquid supply for loosening the soil to be suctioned, thus forming a closed system where the overflow is recycled.
WO2010/122093 also shows an overflow system which uses an overflow pipe which is open at the top for receiving the overflow. The head water enters from the open top and then flows through the vertical pipe. The top of the pipe is adjustable in height to allow more or less flow into the overflow.